Within the papermaking industry, paper or paper board is made by forming an aqueous cellulosic suspension (usually known as a thin stock or paper slurry in general), draining the suspension to form a sheet, and drying the sheet. The draining and drying stages are designed such that the sheet has the desired properties for the final paper or paper board and so generally involves surface treatments to impart adequate smoothness and other performance properties to the sheet. In papermaking, such treatments may involve calendaring for example—i.e., the process of passing paper between the calender rolls to increase the paper's smoothness.
In order to optimize the processes involved in papermaking, it has for many years been standard practice to add various chemical additives to the suspension. Anionic and cationic polymers have been widely used for this purpose. Originally, they were often natural or modified natural polymers, such as cationic starch, but synthetic cationic polymers have been widely used for many years. Their purpose is to act as retention aids and/or as dewatering aids where the given polymer is chosen having regard to the desired property. Such cationic polymer use is common for flat grade machines, while anionic flock polymers are more common in regard to tissue machines. Generally speaking, a retention aid serves to retain fine fibers and fine filler particles in the sheet, whereas a dewatering aid serves to increase the rate of drainage or to increase the rate of drying after drainage. It is well understood that these properties can be mutually conflicting. Accordingly, a large amount of effort has, in recent years, been put into ways of optimizing drainage and dewatering.
The need to improve the quality of the final paper, to avoid loss of fiber or filler fines (e.g., for environmental pollution reasons) and to optimize dewatering (e.g., for energy reduction reasons), means that substantially every significant paper making process has been operated using one or more retention and/dewatering aids. The research into ways for improving these properties has led to the use of different materials in the same process, including the use of sequential addition of different materials.
One such process is described in U.S. Pat. No. 6,048,438 which involves a method to enhance the performance of polymers and copolymers of acrylamide as flocculants and retention aids whereby the flocculation of solid components of the paper mill slurry is increased leading to improved retention of filler and fiber on the sheet and increased drainage of water from the cellulosic sheet produced. Alternative processes exist using the addition of cationic starch followed by colloidal silicic acid. As well, the addition of a synthetic cationic polymer, followed by shearing of the suspension, followed by the addition of bentonite is a process of particular value in the production of fine papers.
The above-mentioned suspensions that are used as the starting material in all these processes, and to which various retention aids and/or dewatering aids are then added, are in all instances made by pulping a fibrous cellulosic material, generally wood. The pulping involves comminution and suspension of the resultant fibers in water. It is generally necessary to wash and filter the pulp several times. The filtering is normally effected by drainage through a screen. A disadvantage of drainage aids is that they tend to increase the amount of thermal drying that is required. In other words, they accelerate the free drainage but they result in the wet sheet containing a larger amount of trapped water, and so additional thermal drying is required. Accordingly, to increase the efficiency of pulp production and, in particular, to increase the rate of production of dry market pulp, requires a reduction in the amount of thermal energy that is required before drying it. This effectively places a balancing act on the judicious use of dewatering aids.
Dewatering processes and dewatering aids are not limited to the papermaking industry. Indeed, the treatment of waste waters, mineral tailings, oily waste waters, municipal and industrial wastes, and the like, will include one or more steps formed by dewatering processes. Often, the goal of such other dewatering processes is extracting liquid from an end-product termed sludge. Specifically, the liquid component is extracted from such solid materials as fibers, colloids, and the like, as the suspension or sludge is deposited onto a moving perforate conveyor belt which acts as a filter. Alternatively, the sludge may be formed by the separation of liquid suspension by means of a centrifugal separator, centrifuge vacuum filter, belt filter press, screw press, or some similar device. Dewatering processes this type are particularly useful in connection with the treatment of sewage sludge, but may also find application in the papermaking industry or in the chemical industry, where the liquid component of a suspension or sludge preparation is to be removed in a continuous operation.
At the end of any industrial process waste treatment system is a mixture of organic solids, ash and water. A system of mechanical dewatering equipment, coagulating/flocculating chemicals and controls algorithms are utilized to dewater the sludge. The ultimate goal of process is to remove the maximum amount of water from the mix to allow the highest sludge dryness. The dryer the sludge, the more options to economically and environmentally dispose of the final waste. Well dried sludge typically has considerable heat value offsetting petroleum fuels use. When the sludge is burned, the volume is reduced by as much as 70% which significantly reduces landfill volumes.
The treatment of sewage sludge has evolved from the previously purely physical separation of its liquid and solid components to prior chemical treatment of the sludge with various conditioning agents such as the colloid producing polyacrylamides. This treatment results in a precipitation effect, which is similar to the flocculation discussed above with regard to papermaking, thus permitting the separation of the water component from the now colloidal sewage prior to mechanical filtration. A conditioning agent is added to a stream of sludge in order to promote the agglomeration of the colloidal particles in the sludge.
Within the input streams of conventional filtering or mechanical dewatering processes there is a sludge stream having suspensions of fibers, colloids, and the like, and a conditioning agent stream having an organic or inorganic material, liquid or solid which, when added to the sludge stream promotes precipitation and or agglomeration of the fine suspensions. Often, a wash water stream is required in certain mechanical dewatering devices to clean the perforate medium and to prevent plugging of the perforations.
Those familiar with dewatering processes recognize that a relationship can be established between a characteristic of the sludge stream, principally the nature and/or the quantity of suspended matter in the liquid carrying medium (i.e., filtrate clarity), and the quantity of (various possible) conditioning agents which may be used to make effective the separation process. The results of the process are a concentrated sludge stream and wastewater. Adjusting the proportion of conditioning agent added to the sludge stream will affect the efficiency of the dewatering process, as measured both by the percentage of total solids contained in the sludge stream which are removed in the concentrated sludge stream, (known as the solids recovery efficiency), and also the proportion of solid matter in the concentrated sludge stream (known as the dewatering sludge “solids content”, typically expressed as “percent dry solids”).
Earlier developments based on improving filtrate clarity measured by a suspended solids meter applied a hill climber scheme with appropriate biasing controls to drive the polymer flow in the desired direction. While such schemes improved filtrate clarity which facilitated sludge drainage thus maximizing final cake dryness, over-drying of the sludge created press plugging and equipment damage.
Those familiar with dewatering processes further recognize that the proportions of conditioning agent and sludge can be controlled in such a manner as to maintain a proportionate relationship between the amount of conditioning agent and the volume and/or solids content of the sludge stream. A typical curve representative of this can be seen in FIG. 7 and applies equally to the principles underlying the present invention. As well, devices can be used to detect the amount of solid matter lost, or passing through, the perforate filtration medium and that this information can, in theory, be used in order to adjust, for example, the volume of conditioning agent added so that an optimum degree of clarity may be achieved in the liquid filtrate passing through the perforate filtration medium.
Such degree of clarity is defined in terms of turbidity or suspended solids. The terms turbidity and suspended solids will be interchangeable throughout this specification. Turbidity is the measurement of the effect that suspended solids has on the transmission of light through an aqueous solution such as water. This is a qualitative measurement where turbidity is measured by shining a light through the water and is reported in nephelometric turbidity units (NTUs). This aspect is seen by way of elements 106 and 409 in prior art FIG. 1 and FIG. 4, respectively. While a turbidity meter 106 as shown may include a single light source to facilitate clarity measurements, a suspended solids meter 409 as shown may include a second light source arranged ninety-degrees from a first light source as suspended solids are less affected by color providing feedback on suspended particles only.
Within FIG. 1 there is shown a simplified schematic of a known dewatering process including a dissolved air flotation device. Gray water (i.e., dirty water produced from some industrial process) is supplied to a tank 101 as effluent from some known industrial process (not shown). A pump 109 moves the gray water through a line to a diffused air flotation device (DAF) 105. In general, a DAF process is a method for separating and removing suspended solids from liquid by attachment of micro size air bubbles to the suspended particles. A flow meter 108 typically resides in the line between the pump 109 and DAF 105 in order to provide information on the rate of gray water flow to the DAF 105. This information is used by a controller 103 that controls the flow ratio of polymer additive relative to gray water. To that end, another flow meter 102 is provided in the polymer feed line so as to provide polymer flow information to the controller 103. The controller 103 uses the information garnered from the flow meters 102 and 108 in order to regulate the polymer flow rate via polymer flow regulator 104 which thereby meters the flow of polymer thus added to the gray water to facilitate flocculation within the DAF 105.
As a product of the flocculation within the DAF 105, waste sludge is separated from the gray water in a manner well known to one skilled in the art of dewatering processes. Removal of the sludge results in a clarified water byproduct having a certain measurable turbidity. A turbidity meter 106 is used to monitor the clarified water byproduct for the desired turbidity readings prior to collection of the clarified water in some form of tank 107. Further processing is possible and is well known within the art of water cleansing and purification to obtain various ranges of water clarity suitable for the given implementation. Typically, an operator will manually set the polymer flow ratio at the controller 103 and monitor the dewatering process by monitoring the properties of the sludge and clarified water products. However, such manual operation is inherently problematic due to the variation in incoming gray water properties coupled with operating variables which necessitate variations in optimum polymer flow required to deliver the optimum filtrate clarity.
Within prior art FIG. 4, there is shown a similar dewatering mechanism involving a sludge press device 417 as mentioned above. In contrast to the dewatering process shown in FIG. 1 which ends as low consistency sludge, the prior art of FIG. 4 begins the dewatering process with relatively aqueous low consistency sludge in storage 401 that is pumped via sludge pump 419 to a flocculation tank 408. A sludge flow meter 403 provides data to a controller 405 which, in conjunction with a polymer flow meter 406, controls the polymer flow ratio by way of a polymer regulator 407 which adds a flocculating polymer to the sludge prior to agitation in the flocculation tank 408. The sludge may then be treated with a pre-thickener within a first stage press 410 whereby a filtrate may be released from the sludge to a filtrate tank 411 and the suspended solids remaining in the filtrate monitored by a suspended solids meter 409.
The pre-thickened sludge is then deposited into the chute 414 of the final stage sludge press 417 which is commonly in the form of a screw press as shown. The level of sludge in the chute is monitored by a chute level transmitter 413 and chute level controller 412 which controls sludge feed flow to the headbox of the screw press to a set point established manually by the sludge press operator. The sludge press 417 is run by an electromechanical press drive 415. The drive 415 itself can be controlled by a sludge feed tank level controller 418 such that the drive speed of the press drive 415 is maintained at a level set point established manually by the sludge press operator. The set point is relative to the level of sludge in sludge storage tank 401 as determined by a sludge tank measurement device 402. Ideally, the established set points for the controllers 405, 412, and 418 provide proper dewatering levels for sludge cakes outputted by sludge press 417 prior to transport and disposal by sludge carriers 416. However, these set points are typically conservatively set to prevent plugging of the sludge press. Such plugging of the press 417 is a function of the sludge dryness as can be seen from FIG. 8. Such conservatism ultimately results in lost throughput potential and reduced dewatering capabilities.
Known control systems which depend on detecting changes in filtrate turbidity do not allow for the fact that turbidity changes can occur for a number of reasons. The chemical and/or the physical make up of the sludge has continually varying properties, such as PH swings, primary to secondary solids ratio, sludge temperature, polymer effectiveness and others. These normal variations in operating conditions lead to significant variations in the filtrate turbidity. Less than optimal filtrate clarity translates—to less than optimal first pass solids retention (i.e., optimal drainage) which translates—to less than optimal sludge cake dryness.
A disadvantage of existing control systems is that, as the various sludge properties change, the required polymer feed rate to achieve optimum clarity requires change. Optimum filtrate clarity can be substantially different pending the type and magnitude of sludge property variances. Equipment suppliers have manufactured many different types of dewatering devices such as screw presses, belt presses, rotary thickeners and dissolved air floatation devices. Chemical suppliers manufactured polymers used to bond the sludge particles which enhanced dewatering. The missing element has been an overall control system to regulate the polymer feed and press feed rate that would allow the maximum sludge cake dryness while protecting presses from plugging. In some instances, a press drive current control scheme has been used to maximize allowed sludge press feed level while attempting to protect the press from plugging. To date, such solutions have not yielded optimum performance.
Within industrial dewatering processes in general, it is therefore desirable to provide improvements to minimize the dewatering agents used while maximizing process throughput and increasing sludge dryness without detrimental side-effects. Within the clarification processes (e.g., dissolved air floatation, flocculation, and the like), it is desirable to attain the most pure effluent filtrate clarity while minimizing the least amount of flocculant additives.